To enhance the permeability of reservoir rock, hydraulic fractures are stimulated by injecting fluid and proppant into the rock matrix. The resultant stress perturbation induces microseismicity in the formation, which can be recorded by properly positioned geophones. Microseismic data may be acquired during hydro-fracture treatments to validate and assist completions; specifically, engineers would like to use the data to (a) determine the dimensions of the failure and its relation to hydraulic fracture treatment data, and (b) determine the interaction between perforation stages so as to optimize perforation spacing. Additionally, engineers would like to use the data to assist in landing wells in the formation and to illuminate faults and potential fault re-activation.
Hydraulic fracture modeling is used to determine the volumes and rates of fluid injected into the subsurface. The models use best estimates of elastic parameters of the rock, and calculate the volume and rate of fluid and proppant to be injected to create a fracture of certain dimensions. Microseismic monitoring is one of the few methods that allow us to determine the actual volume of rock that was fractured.
Current methods of analyzing microseismic data use magnitude (a compressed dynamic range) to represent microseismic event size. These methods do not distinguish between microseismic events that are related to the hydrofracture and events triggered on nearby faults for purposes of calculating fracture dimensions. Values assigned to fracture dimensions (used in Stimulated Rock Volume calculations) are based on arbitrary shapes drawn around all detected microseismicity. Even when the full dynamic scale (usually dynamic amplitude) is used to quantify the event size, this measure is not used in quantifying fracture height growth and wing length. This results in a gross over-estimation of fracture dimensions and poor use of microseismic data.
In an article by Wessels et al. (“Identifying faults and fractures in unconventional reservoirs through microseismic monitoring,” First Break 29, 99-104 (2011)), the authors describe how in microseismic monitoring of low permeability reservoirs, the use of source mechanism inversion, b values, and energy release rates enables identification and differentiation between fracture stimulation and fault activation, critical issues for effective hydraulic treatment. This paper uses the energy of microseismic events to distinguish between events that are related to stimulated fractures from events that are triggered on a pre-existing tectonic fault. Fracture dimensions are not discussed.
U.S. Patent Application Publication No. 2009/0299637 (“Continuous Reservoir Monitoring for Fluid Pathways Using Microseismic Data,” S. N. Dasgupta; see claim 7) discloses a method in which the computer compares the detected pathways for preferential fluid movement in the reservoir with predetermined computational models of the pathways. The dimensions of the fracture are not computed. In U.S. Pat. No. 6,947,843, to Fisher et al. (“Microseismic signal processing”), the dominant source frequency is used to determine the radius of the fracture plane, the polarity of the waveforms to determine the orientation of the fracture plane, and waveform similarity and proximity of events to define hydraulically connected volumes. The objective of finding the hydraulically stimulated volume is the same, but the approach is very different from the present inventive method. U.S. Pat. No. 7,460,436 to Segall and Sang-Ho (“Apparatus and method for hydraulic fracture imaging by joint inversion of deformation and seismicity”) discloses a method to image hydraulic fractures by using ground deformation and seismicity data and minimizing weighted sums of the differences between observed and predicted values of the two measurements. The present inventive method does not use ground deformation and is not model based. The method presented in “Evaluating Hydraulic Fracturing Success in Tight Gas Formations: A Seismic view,” by Mark Willis, pp. 1-7, downloaded from http://www.seismicmicro.com/PDF/willis_hydraulic_fracturing_wp_smtwc052108.pdf, uses a combination of scattered energy and VSP data to image and characterize fractures. The present inventive method does not use the energy scattered from the fractures.
In summary, current methods of estimating hydraulic fracture dimensions are either model based or based on datasets where all the microseismic events, regardless of event size and/or recording bias, are used. In contrast, the present invention is a data-based method of calculating hydraulic fracture dimensions using a systematic approach to select microseismic events from all the recorded events. The method also minimizes the effect of recording bias on the estimates.